1. Field
The invention relates to holders for punching tools used with turrets and punch blocks, and more particularly to means for extending the usable life of the associated toolholder guides.
2. State of the Art:
Repeated use of punch tools results in wear of the bores for guiding and positioning the toolholders, punching operations eventually becoming impractical because the toolholder is no longer sufficiently guided and centered with respect to the workpiece. The wear on the bore is typically not uniform, so that replacement of the toolholder with one of larger diameter is not practical. Restoring the guiding bore to usable condition usually requires it be re-bored, which requires replacement of the toolholder with one of appropriately larger diameter. Thus, both the bore and the toolholder must be refurbished. The resulting toolholder is of non-standard size, leading to confusion in use. Re-boring of the toolholder bore generally requires complete dismantling of the turret punch press to send the turret to another location or facility for boring. It is very desirable that some method be available to refurbish the worn bore that permits the use of the original toolholder and avoids the lost time and cost of dismantling and re-boring. Equally desirable is a toolholder guide capable of long life while being easily repaired. One method of refurbishment is disclosed in U.S. Pat. No. 4,240,314. This method involves the drilling of spaced side bores into the toolholder wall along with the insertion of plugs of resilient material into the bores to position the toolholder away from the wall. The plugs extend outwardly into the toolholder bores so that the toolholder compresses the resilient plugs tightly into the side bores. The toolholder reciprocates across the ends of the plugs. The small plugs provide limited bearing surface, and wear soon occurs requiring replacement of the plugs, although their resilient springback extends the useful life somewhat. The plastic tends to creep under constantly applied load so that the advantage of the compression is largely lost. This method is therefore of severely limited value, either for repair or for original equipment. A more desirable and practical method for refurbishment is disclosed in U.S. Pat. No. 4,756,630. With this method, the walls of the toolholder bore are broached longitudinally to provide keyways, into which are installed plastic wear strips protruding beyond the walls into the bore. The innermost surfaces of the strips are broached to a close clearing radius with the body of the toolholder so that the toolholder is accurately centered and guided for reciprocal motion within the bore. The broaching operations are carried out using a broaching fixture which secures an actuating hydraulic cylinder directly to the punch block or the turret, using adjacent toolholder bores for attachment, avoiding the necessity of dismantling the turret. This method provides a greatly improved toolholder guide capable of considerably extended life. However, the wear strips themselves eventually wear and require periodic replacement. A need therefore remains for a practical method of refurbishment of toolholder guides, especially in turret punches and punch blocks, further extending the usable life of the toolholder guides beyond the present methods.